1. Field of the Invention
The present invention relates in general to four wheel drive vehicles and more particularly to a device for controlling driving force distribution in a four-wheel drive automotive vehicle.
2. Description of the Prior Art
A prior art device for the above described control is disclosed in the Japanese Provisional Patent Publication No. 61-157437.
The prior art device is constrcuted so as to calculate the rotational speed differential between the front and rear wheels based on signals produced by the front and rear wheel rotational speed sensors and to increase the clutch engaging force as the rotational speed differential between the front and rear wheels increases, i.e., as the driving wheel slip increases, whereby to change the driving mode for the four-wheel drive and prevent the driving wheel slip promptly. Further, the prior art device is constructed so that its control constant causes the steering characteristic during cornering to become neutral steer when the vehicle speed ranges from low to middle.
The prior art device has the following problems.
(1) The running stability of the vehicle at the time of lane change or when subjected to a small disturbance is rather bad since the clutch is brought into engagement with some delay after occurence of the rotational speed differential between the front and rear wheels even during high speed straightahead running of the vehicle.
(2) While weak understeer is desirable during high speed cornering, the cornering or turning characteristic effected by the prior art device is neutral steer, thus resulting in that the driver needs to take good care in operating the steering wheel during high speed cornering.
(3) If a greater importance is given to the control of the clutch engaging force upon high speed running in order to solve the above problems (1) and (2), the running stability during low to middle speed running is deteriorated.